1. Field of the Invention
The present invention relates to a substrate for use in a liquid crystal display, a method of manufacturing the same, and a liquid crystal display using the same, and particularly to a substrate for use in a liquid crystal display, which is used for an active matrix type liquid crystal display using a switching element of a thin film transistor (TFT) or the like, and a method of manufacturing the same. Further, the invention relates to a substrate for use in a liquid crystal display of a CF-on-TFT structure in which a color filter (CF) is formed on the side of an array substrate on which a switching element is formed, a method of manufacturing the same, and a liquid crystal display using the same.
2. Description of the Related Art
An active matrix type liquid crystal display (LCD) using a TFT as a switching element is disclosed in, for example, Japanese Patent Laid-Open No. 202153/1994. The publication discloses, as outlined below, a reversed stagger type TFT-LCD in which a channel protection film is formed.
A passivation film made of an inorganic insulating material is formed on almost the whole surface of a substrate on which a TFT is formed. A pixel electrode formed of a transparent electrode material is formed on the passivation film. The pixel electrode is connected to a source electrode of the TFT through a contact hole opened in the passivation film.
An external connection terminal (hereinafter abbreviated to a drain terminal) connected to a drain bus line includes a lower electrode formed of an n+-type a-Si layer and a metal layer. An upper electrode made of an oxide conductive film of the same material as the pixel electrode is laminated on the lower electrode through the contact hole opened in the passivation film so that oxidation of the lower electrode is prevented. A connection terminal of a drain bus line driving circuit is connected to the upper electrode, and a predetermined gradation voltage is applied to the respective drain bus lines.
An external connection terminal (hereinafter abbreviated to a gate terminal) connected to a gate bus line includes a lower electrode formed of a metal layer constituting a layer common to a gate electrode and the gate bus line. An upper electrode made of an oxide conductive film of the same material as the pixel electrode is laminated on the lower electrode through a contact hole opened in an insulating film constituting a layer common to a gate insulating film and in the passivation film, so that oxidation of the lower electrode is prevented. A connection terminal of a gate bus line driving circuit is connected to the upper electrode and a predetermined gate pulse is sequentially applied to the respective gate bus lines.
Next, a method of manufacturing the reversed stagger type TFT-LCD in which the channel protection film is formed will be outlined. A plurality of gate bus lines and gate terminal lower electrodes are formed on a transparent insulating substrate such as a glass substrate. Next, an insulating film is formed on the whole surface. Incidentally, in this insulating film, its part on the gate electrode is especially called a gate insulating film. Subsequently, an a-S Si layer is formed on the insulating film, and next, a channel protection film is formed. Next, after an n+-type a-S Si layer is formed, a metal layer is formed, and etching is collectively carried out while using the channel protection film as an etching stopper, so that an active semiconductor layer of the a-Si layer is formed on the gate insulating film of a TFT portion, and a source electrode and a drain electrode are formed at both sides of the channel protection film, whereby a TFT is completed.
Besides, at the same time, the drain terminal lower electrode connected to the drain bus line and made of the n+-type a-Si layer and the metal layer is formed.
Next, a passivation film made of an inorganic insulating material, such as an SiN film, an SiO2 film, or a composite film of these, and having a thickness of 400 nm is formed on the whole surface. Next, after a resist is coated, a photolithography method is used to form a resist pattern having openings over the source electrode, the drain terminal lower electrode, and the gate terminal lower electrode. Then, the passivation film or the passivation film and the insulating film are etched using the resist pattern as a mask, so that contact holes are opened.
Next, a transparent conductive film having a thickness of 100 nm and made of ITO or the like is formed on the whole surface by using a sputtering method or the like. Next, the transparent conductive film is patterned into a predetermined shape, and the pixel electrode connected to the source electrode through the contact hole is formed. At the same time, the drain terminal upper electrode connected to the drain terminal lower electrode through another contact hole is formed, and the gate terminal upper electrode connected to the gate terminal lower electrode through still another contact hole is formed.
As stated above, according to the description of the above publication, in the case where the gate terminal and the drain terminal are formed, the gate terminal lower electrode and the drain terminal lower electrode are formed, the passivation film covering the upper portions of the gate terminal lower electrode and the drain terminal lower electrode is formed, the passivation film is etched to open the contact holes, and the gate terminal upper electrode made of the transparent conductive film connected to the gate terminal lower electrode through the contact hole and the drain terminal upper electrode made of the transparent conductive film connected to the drain terminal lower electrode are formed simultaneously with the pixel electrode.
Japanese Patent Laid-Open No. 2000-231123 discloses that an edge portion of a color filter formed for each pixel is overlapped with a light shielding film (black matrix; BM) for shading a storage capacitance bus line intersecting a region between pixels and the inside of a pixel against light, when viewed in the direction of a normal of a substrate surface.
Further, Japanese Patent Laid-Open No. 092022/1979 discloses that a light shielding film is provided on an array substrate or an opposite substrate in order to suppress the generation of leak current due to the photoconductivity of a TFT.
Furthermore, with respect to a CF-on-TFT structure in which a color filter is formed on the side of an array substrate, for example, Japanese Patent Laid-Open No. 140324/1981 discloses that spectroscopic characteristics between adjacent pixels are different and color filters between pixels are stacked to form a columnar spacer.
Besides, in the CF-on-TFT structure, it is well known that a light shielding function is provided by stacking color resins, a wiring line is used as a light shielding film to ensure an opening ratio, and a color resin is overlapped with an edge of a light shielding film.
Besides, Japanese Patent Laid-Open No. 068726/1989 discloses that a pixel is formed on a flattened transparent insulating film which is thin in a region over a TFT and is thick in other regions.
Further, a frame region at an outer periphery of the display region formed by a plurality of pixels arranged in a matrix form is required to function as a light shielding region to block out the leakage light from a backlight. Therefore, in case of an LCD in which CF is formed on the side of the opposite substrate, the light shielding function is provided by laminating a resin CF layer in the frame region or by forming a low reflection Cr(chrome) film. Further, in the LCD with the CF-on-TFT structure, the light shielding function is provided by laminating the resin CF in the frame region.
A case will be considered in which an overcoat (OC) layer of an insulating organic resin material is used instead of the passivation film of the inorganic insulating film disclosed in the above publication of Japanese Patent Laid-Open No. 202153/1994. The film thickness of an inorganic insulating film, such as a silicon nitride film (SiN), is generally as thin as 300 to 400 nm, whereas the OC layer has a feature that its film thickness becomes very thick, for example, 1000 to 3000 nm. Besides, the dielectric constant of a resin forming the OC layer is about 3 or less and is relatively small, and in combination with the large film thickness, the OC layer has a merit that parasitic capacitance to deteriorate TFT characteristics can be reduced.
On the other hand, the OC layer is inferior to the passivation film made of the inorganic insulating material in adhesion to an electrode material formed on an upper layer, and a large stepped portion is formed since the film thickness is large. Thus, defective conduction due to discontinuity of the electrode material formed on the upper layer is apt to occur, and a problem of poor etching, such as formation of a residual substance of the electrode material or the occurrence of thinning of an electrode width, is apt to occur.
Further, also in the case where a contact hole is opened in the OC layer to electrically connect the lower electrode, it becomes necessary to sufficiently consider the shape of the contact hole formed in the thick resin layer of the OC layer or the positional relation between the hole position and the upper and lower electrodes.
Moreover, there also arises a problem that in an inspection process of a wiring pattern formed on a substrate for use in a liquid crystal display, since the film thickness of the resin CF layer is large, it is difficult to focus a measurement device equipped with an incident-light optical system on the wiring pattern of the lower layer of the resin CF layer.
Also, when a BM film is used for block out light at the frame region, a low reflective Cr film or a black resin film is ordinary used as a BM film. However, these formation processes result in a high cost in panel production. When a shield layer is formed by superposing the resin CF layers, there is a laminated structure of three layers of R, G and B or a two-layer structure of the CF resin layers using also the shielding function of the liquid crystal layer to raise the shielding function. However, a problem of light leakage can be produced.
Besides, in the CF-on-TFT structure, in the case where for example, a resin in which a pigment is dispersed as a color component is used for the CF layer, attention must be given to a possibility that an inorganic component of the pigment pollutes a liquid crystal layer and a semiconductor layer. According to the CF-on-TFT structure, basically, it is sufficient if only a common electrode and an alignment film are formed on the side of the opposite substrate, and simplification of the substrate can be realized. However, since the light shielding function conventionally provided on the side of the opposite electrode is also omitted, it becomes an important problem how to cause the array substrate to have the light shielding function optimally.
With respect to the light shielding function of the CF-on-TFT structure, there arise a problem of an erroneous operation due to the photoconductivity of the TFT through the incidence of external light such as room light or sun light, and a problem of the glare of a peripheral frame portion due to leakage light from a backlight in a transmission type display and a drop in contrast of a pixel. With respect to the frame portion of the periphery of a display region, it has been clarified from experimental results that since the light of the backlight is high, it becomes necessary to provide a light shielding film in which resin CF layers of at least two colors are laminated. However, when two layers of the resin CF layers are laminated in the frame portion, and the CR layer of one layer is formed in the pixel of the display region, there arises a problem that the height of the display region becomes different from that of the frame portion, and the cell gap thickness becomes uneven. Even if the OC layer is formed on the whole surface to flatten, since the height of the laminated resin CF layer 42 of the frame portion is relatively large, a sufficient flattening effect can not be obtained.